Method and apparatus for treating a web

ABSTRACT

An improved method and apparatus for substantially eliminating non-uniform charge patterns from a surface of a corona-treated web, particularly for enhancing the surface&#39;&#39;s coatability, is disclosed. The improved method and apparatus employs the technique of &#39;&#39;&#39;&#39;flooding&#39;&#39;&#39;&#39; the surface with charge of one polarity from a first electrode so as to generally uniformly charge the surface, and thereafter removing the charge imparted to the surface by applying an equal but opposite charge from a second electrode. To ensure positive control between the amount of charge added to the surface and the amount removed, the invention proposes that the first and second electrodes be connected to a single source of DC power and that the web, when treated in accordance with the invention, be supported by an electrically conductive roller which is electrically isolated from ground.

United States Patent [191 Kerr Apr. 24, 1973 METHOD AND APPARATUS FORPrimary ExaminerL. T. HiX TREATING A WEB Attorney-'W. T. French et al.[75] Inventor. William C. Kerr, Middlesex, NY. ABSTRACT [73] Asslgnee'Eastman Kndak Company An improved method and apparatus for substantiallyochester, N.Y.

eliminating non-uniform charge patterns from a sur- [22] Filed: Sept.30, 1971 face of a corona-treated web, particularly for enhancing thesurfaces coatability, is disclosed. The im- [211 Appl' NO" 185,023proved method and apparatus employs the technique 4 of flooding" thesurface with charge of one polarity US. Cl. R, A, from a first electrodeso as to generally uniformly 204/312 charge the surface, and thereafterremoving the Int- C]. u, charge to the urface an equal [58] Field ofSearch ..317/2 R, 2 A, 262 R, but opposite charge from a Secondelectrode To 317/262 A? 204/312 sure positive control between the amountof charge added to the surface and the amount removed, the in- [56]References Cited vention proposes that the first and second electrodesUNITED STATES PATENTS be connected to a single source of DC power andthat the web, when treated in accordance with the inven- 3,281,34710/1966 Winder "204/16 8 tion, be supported by' an electricallyconductive roller 3 9 2/1968 Wood which is electrically isolated fromground. 3,531,314 9/l970 Kerr et al. ..204/3 l2 9 Claims, 1 DrawingFigure Patented April 24, 1973 mm Mm WILL/AM c. KERR INVENTOR. BY W W?ATTORNEY METHOD AND APPARATUS FOR TREATING A WEB CROSS-REFERENCE TORELATED APPLICATIONS This application is related to U.S. applicationSer. Nos. 804,470, filed Mar. 5, 1969, and 167,758, filed July 30,1971.

BACKGROUND OF THE INVENTION This invention relates to the treatment ofwebs. More particularly, the invention relates to a method for treatinga web so as to improve the webs coatability.

In the prior art, it is common practice to coronatreat a surface of aweb to improve the surfaces receptivity to inks, adhesives, photographicemulsions, subbing layers, etc. For example, a web of photographic papermay be polyethylene-coated; and such coating may be corona-treatedbefore the application of a photographic emulsion thereon. Whilecorona-treatment of the polyethylene surface may enhance the receptivityof the surface to the emulsion, a problem arises in thatcorona-treatment of the surface usually leaves a nonuniform distributionof, i.e., a pattern of, electrostatic charge on the surface. In coatingsystems where the coating operation is delicate, i.e., easily disturbed,this charge pattern must be substantially removed before the web isintroduced into the coating station, if uniform coatings are to beproduced. Otherwise, the charge pattern will cause coating beaddisturbances and thus cause the coating to be uneven.

Apparently, in the corona-treatment of the polyethylene surface byconventional alternating-current corona-producing means, the alternatingelectric field created by the corona-producing means distributes chargedparticles of different polarities onto the surface in a non-uniformcharge pattern. Thus, along the surface there may reside hillscomprising a relatively large amount of positive charge, and adjacentthereto valleys of a relatively large amount of negative charge. As theemulsion coating is deposited on the web, it is affected by the surfacecharge distribution, and in a sense the emulsion aligns with the chargepattern and forms what has been called crosslines, thus adverselyaffecting the photographic quality of the product in question.

A need has developed, therefore, to remove charge from a surface to becoated, or at least to reduce the charge, or the effect of its pattern,to an acceptable level before coating the surface with photographicemulsions.

The problem of removing the non-uniformly distributed charge from thecorona-treated surface is believed to be complicated by the nature ofsuch charge. The charge may be comprised of two types:

1. free charges on which the mating charges of opposite polarity areremote from the web, and

2. polar or bound charges in which charges of opposite polarity are onthe opposite side of the web.

Prior art methods of removing surface charges from the web work quitewell when there are only free charges to be removed. However, polar orbound charges are extremely difficult to reduce to a satisfactory level.

One technique for removing the non-uniform distribution of charge on acorona-treated polymeric surface is described in U.S. Pat. No.3,531,314. In this patent, a corona-treated web is advanced to acharging station, wherein a charged roller floods the hills" andvalleys" of surface charge and imparts a highly uniform charge level toone surface of the web. Simultaneously, a counter electrode positionedon the other side of the web imparts a highly uniform charge of oppositepolarity to the other side of the web. Thereafter, both sides may bepassed over grounded rollers, or wound into rolls, to reduce the levelof the charge to below arc-threshold so that a photographic orlightsensitive emulsion may be coated on one of the sides withoutsubstantial fogging of the emulsion. Obviously, winding of the web intorolls is undesirable where one wishes to corona-treat and coat in acontinuous operation. The use of grounded rollers to dissipate the highlevel of charge imparted to both sides of the web while removing much ofthe charge still leaves a relatively high level of (bound) charge on thesurface.

An improvement upon the technique recited in the afore-mentioned patentis described in U.S. application Ser. No. 804,470, filed on Mar. 5,1969. This improvement employs the flooding technique disclosed in U.S.Pat. No. 3,531,314, but includes a series circuit feature wherein firstand second electrodes such as rollers are connected to the oppositelycharged terminals of a source of DC potential and are located on theface side of the web. Counter electrodes for the rollers are located onthe other (hereinafter called the wire) side of the web; the counterelectrodes being electrically connected to each other. In operation, ata first charging station the first roller applies, say, a large amountof positive charge to the face side of the web and induces its counterelectrode to deposit an equal amount of negative charge onto the wireside of the web. Theapplication of negative charge is able to neutralizethe polar positive charge on the wire side, and thereby frees the polarnegative charge on the face side, which may then be either conductedaway or neutralized by the positively charged roller. The web is thenconveyed to a second charging station comprising a relatively negativelycharged roller and its relatively positively charged counter electrode.At this point, positively charged particles are applied to the wire sideof the web to neutralize the negative charge, thereby freeing thepositive charge on the face side which may then be removed by therelatively negatively charged roller. One important feature of thistechnique is that because of the series circuit feature, the amount ofcharge applied to each surface at the second charging station is equalto the amount of charge of opposite polarity applied to each surface atthe first charging station.

While the apparatus and method described in the aforementionedapplication Ser. No. 804,470 generally gives satisfactory results, aproblem arises in that when the web is conveyed about the two rollerswhich are charged relative to the web, the web has an undesirabletendency to adhere to the rollers and, thus, creases are likely to formin the web. A reduction in the number of relatively charged rollersabout which the web is conveyed represents a highly desirableimprovement.

SUMMARY OF THE INVENTION The invention provides an improved method andapparatus for use in removing non-uniform charge patterns from acorona-treated web. The invention employs the series circuit featuredescribed in the aforementioned application Ser. No. 804,470, i.e., twoelectrodes which are exposed to one side of a web are connected to theopposite poles ofa source of DC potential for first uniformly chargingthe said one side with charge of a first polarity and for thereafterremoving the uniform charge. The invention improves upon this feature byproviding an electrically conductive support member about which theopposite side of the web may be wrapped; the electrically conductivesupport member is electrically isolated from ground and is so disposedas to comprise a common counter electrode for both said electrodes.

The series circuit of the invention may be described, in effect, as theflow of current from the DC source to one electrode, then through theweb to the electrically conductive support member, and back through theweb to the second electrode and to the DC source. By the termelectrically conductive support member, it is meant that, with the useof the support member in the series circuit of the invention, theeffective electrical resistance of the circuit is no more than aboutohms, i.e., the potential of the source divided by the current flowingin the series circuit is no greater than about 10 ohms.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, there is shown arepresentation of an apparatus for carrying out the improved method andapparatus described herein.

DESCRIPTION OF THE PREFERRED EMBODIMEN'IYS With regard to the drawings,a coating apparatus is shown wherein a web 10 of dielectric material orone having a dielectric coating is conveyed from an unwind station 13 toa corona-treatment station 15, hereafter referred to as Zone 1. As usedherein, the term dielectric refers to any surface with a surfaceresistivity of about 10 ohms per square or greater. At Zone 1, the web10 is conveyed about an electrically grounded roller 17 so that one websurface, 12, is in intimate contact with the surface of the roller. Onthe opposite side of the web, web surface 11 is exposed to aconventional alternating-current corona-treatment, so as to enhance itsreceptivity to a coating, such as, for example, inks, adhesives,photographic emulsions, subbing layers, etc. In corona-treatment of thesurface, a glow of corona may be seen to extend from electrodes 21,23,and 25 to the surface 11. The electrodes may be connected to a suitablealternating-current power supply 31 for providing a suitable level ofpotential for ionizing the air between the electrodes and the surface.As used herein, alternating-currents and voltages are any such currentsand voltages which vary, such, for example, as sinusoidal and pulsatingcurrents and voltages. If desired, provision'may be made in Zone 1 forcoronatreating web surface 12.

After corona-treatment, the dielectric surface becomes highly chargedwith charge that is, as indicated in the drawing, non-uniformlydistributed along the surface 11. To remove this charge, which may becomprised of both polar and free charges, the web is conveyed to Zone 2.At Zone 2, the web 10 is wrapped about a metal roller 27 which iselectrically isolated from, i.e., virtually floated, with respect toground. The

web surface 12 is in intimate contact with the conductive surface 28 ofthe roller 27. While the roller 27 is preferably a metal roller, theroller 27 may include a covering of dielectric material with the provisothat, as stated above, the total effective electrical resistance in theseries circuit should be no greater than about 10 ohms.

The web surface 11 is exposed to first and second electrodes 29 and 37,respectively; and which electrodes are spaced from the web surface 1 1 adistance of about, say, l to l.5 centimeters. The first electrode 29 islocated proximate to roll 27 and is connected to one pole of a source 33of DC potential. The source 33 is preferably of the order of 25 to 30 Kvor more, and draws about 2 m.a. of current. Thus, the effectiveelectrical resistance in the preferred embodiment is about, say, 15megohms. While the source 33 may be either of negative or positivepotential, the discussion of the preferred embodiment will assume it isnegative. The second electrode 37 is located downstream of the firstelectrode 29. The second electrode 37 is located proximate to the roll27 and is connected to the positive pole of the source 33. It will benoted from the drawing that such connection may be through ground.

The conductive roller 27 may be electrically connected to athree-position switch 46. With the switch 46 coupled to a terminal 47,the roller 27 is biased negatively by being coupled through a rheostat50 to negative electrode 29. Conversely, when the switch 46 is coupledto a terminal 49, the roller 27 will be biased relatively positively bybeing coupled through a rheostat 51 to the positive electrode 37. Whenswitch 46 is coupled in the third position, i.e., coupled to terminal48, the switch 46 will be open and roller 27 will be insulated fromground. In each of the three switch positions, the roller iselectrically isolated from ground; that is, the roller 27 will be eitherelectrically insulated from ground or connected to ground through a highimpedance.

The operation of the apparatus in Zone 2 is as follows: Electrode 29,being highly negatively charged, ionizes the air in the vicinity of theelectrode and deposits or floods surface 11 with a large amount ofnegative charge. The flood of negative charge eliminates the non-uniformcharge pattern applied to the web during corona-treatment of the web inZone 1, and provides thesurface 11 with a high negative charge. Itshould be appreciated that while a corona glow appears about theelectrode 29, such glow does not, and preferably should not, extend tothe surface 11, as it would during conventional corona-treatmentoperations.

Simultaneously with the application of negative charge to the surface11, the roller 27 functioning as a counter electrode for electrode 29applies to the surface 12 a high positive charge which is equal butopposite in polarity to the charge on the surface 11. The positivecharging of the side 12 by the roller 27 may be either by the depositingof positively charged ions or particles onto the surface 12 or by theremoval of electrons from the surface 12. Since the roller 27 isisolated from ground, a negative potential will be induced upon theroller by the electrode 29. The magnitude of this potential may beapproximately one-half that of the source 33 with switch 46 positionedin the open'position, i.e., connected to the terminal 48. As the websurface 11 comes between the roller 27 and the grounded electrode 37,the potential difference between the roller 27 and the electrode 37induces the electrode 37 to strip electrons from the air in the vicinityof the electrode 37, thereby causing'positively charged ions orparticles to be deposited on the surface 1 1 to neutralize the negativecharge applied on the surface by electrode 29. Thus, bound charges areremoved from the surface 11 of the web. Again, itshould beappreciated'that while a. corona glow may appear around the electrode37, the glow does not, and preferably should not, extend to the surface11. With the removal of substantially all of the bound charge from thesurface 11, the bound positive charge on surface 12 is eliminated bydepositing electrons or negatively charged ions onto the surface 12 bythe roller 27. Preferably after the web is separated from the roller 27,any charge remaining on the surface 11 may be removed by a groundedelectrode 41. Thereafter, the web may be conveyed about groundedconveyance rollers 43 to a coating station 63, wherein one or morecoatings 65 may be deposited on the web.

An important feature of this invention, like that of application Ser.No. 804,470, is that the amount of negative charge deposited bythe'electrode 29 on the surface 11 must be the same as the amount ofpositive charge deposited thereon by the electrode 37. Since there isbut one source of potential source 33 and since the two electrodes areconnected to opposite poles of this source with switch 46 in the openposition, any electrons which are stripped from the air by the electrode37 are put back into the air by the electrode 29. Thus, from the above,it will be appreciated that the balancing of the charge applied andremoved from the web by the two electrodes is inherent in theiroperation. However, as stated previously in the discussion relative toapplication Ser. No. 804,470, there are instances where one electrodemay deposit more charge than the other can remove. In these instances,such as where an electrically conductive layer comprises one layer ofthe web, it may be necessary to bias the roller 27 either positively ornegatively by positioning the switch 46 to the positions 49 or 47,respectively. If it is found that an excess of negative charge is beingplaced on the surface 11, then the roller may be negatively biased so asto reduce the potential difference between the roller 27 and electrode29. Conversely, if an excess of positive charge is being placed on theweb, then the roller 27 may be positively biased so as to reduce thepotential difference between the roller and electrode 37.

It should be appreciated that while the drawing shows the surface beingexposed to electrodes 29 and 37 as the surface which is to be coated,this is not a necessity, as the surface which contacts the roller 27will also have a non-uniform charge pattern removed from such surface inthe Zone 2 treatment described above.

The invention has been described in relation to preferred embodimentswherein a web has received a non-uniform distribution of charge on itssurface(s) during a corona-treatment process; however, in its ticularcoatings to be placed thereon. The method for carrying out the inventionmay be quite useful in the treatment of webs comprised of films andsurfaces of various materials, including polyolefins such aspolyethylenes, polypropylenes, and ethylene-propylene copolymers,polystyrene, polybutenes, polypentenes,

polyacrylic acid esters, linear polyesters and polycarbonates such aspolyethylene terephthalates, polyamides such as nylon, cellulose esters,polyacrylonitrile, polyvinylidene chloride and other copolymers of theindicated monomers, such as ethylene-vinyl acetate copolymers.Although'it has special advantages in connection with photographicproducts, coatings other than the preferred light-sensitive silverhalide gelatin emulsions may be placed on the web. Thus,non-lightsensitive gelatin emulsions, as well as other compositionswhich 'may becoated by a hopper, such as coatings including magneticoxides, organic solvents, etc, may be coated in accordance with theinvention described herein. a

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:

1. An apparatus for treating a moving web, which apparatus comprises:

a. an electrical source of DC potential,

b. a first electrode means coupled to one pole of said source, saidfirst electrode means being positioned in facing relationship to oneside of said web so as to generally uniformly electrically charge thesaid one side, and v c. a second electrode means coupled to an oppositepole of said source, said second electrode means being positioned infacing relationship to said one side of said web so as to substantiallyremove the charge imparted to the said one side by said first electrode;and the improvement which comprises:

d. means for supporting said web, said supporting means including anelectrically conductive support member which is isolated from ground andwhich is adapted to contact the opposite side of said web, said memberbeing charged by said. source to a potential between those applied tosaid first and second electrodes, and said member cooperating with saidelectrodes so as to comprise a common counter electrode for saidelectrodes.

2. The invention according to claim 1 wherein the first and secondelectrode means are spaced from said one side of said web, and saidsupport member is comprised ofa roller.

3. The invention according to claim 1, in combination with apparatus forcorona-treating the web, the corona-treating apparatus being adapted tocoronatreat the web at a location upstream of the support member.

' 4. The invention according to claim 3, in combina: tion with means forcoating the web, the coating means being adapted to coat the web at alocation downstream of the, support member.

5. The invention according to claim 1, wherein said electricallyconductive support member is insulated from ground.

6. The invention according to claim 1, wherein a switch is electricallyconnected to the electrically conductive support member, the switchincluding at least three positions which comprise:

a. means to bias the support member to a first pole of the DC source,

b. means to bias the support member to a second pole of the DC source,and

c. an open position wherein the support member is insulated from ground.

7. A process for the preparation of a web for deposition of a coatingthereon, the web including a nonuniform distribution of electric chargethat is to be substantially removed from the web prior to coating, theprocess including the steps of:

a. providing an electrical source of DC potential, the

source having first and second electrodes with the DC potential appliedbetween the electrodes;

b. exposing one side of the web to the first electrode so as togenerally uniformly electrically charge the said one side; and

c. exposing the said one side to the second electrode so as to removethe charge imparted to the said one side in step (b), and theimprovement whic comprises:

d. supporting the web so that the opposite side thereof is in contactwith an electrically conductive support member that is isolated fromground, said support member being charged by said source to a potentialbetween those applied to said first and second electrodes, whereby saidsupport member comprises a common counter electrode for said first andsecond electrodes.

8. The invention according to claim 7 wherein a side of said web is adielectric.

9. The invention according to claim 8 wherein the non-uniformdistribution of electric charge was placed on the web during acorona-treatment process.

1. An apparatus for treating a moving web, which apparatus comprises: a.an electrical source of DC potential, b. a first electrode means coupledto one pole of said source, said first electrode means being positionedin facing relationship to one side of said web so as to generallyuniformly electrically charge the said one side, and c. a secondelectrode means coupled to aN opposite pole of said source, said secondelectrode means being positioned in facing relationship to said one sideof said web so as to substantially remove the charge imparted to thesaid one side by said first electrode; and the improvement whichcomprises: d. means for supporting said web, said supporting meansincluding an electrically conductive support member which is isolatedfrom ground and which is adapted to contact the opposite side of saidweb, said member being charged by said source to a potential betweenthose applied to said first and second electrodes, and said membercooperating with said electrodes so as to comprise a common counterelectrode for said electrodes.
 2. The invention according to claim 1wherein the first and second electrode means are spaced from said oneside of said web, and said support member is comprised of a roller. 3.The invention according to claim 1, in combination with apparatus forcorona-treating the web, the corona-treating apparatus being adapted tocorona-treat the web at a location upstream of the support member. 4.The invention according to claim 3, in combination with means forcoating the web, the coating means being adapted to coat the web at alocation downstream of the support member.
 5. The invention according toclaim 1, wherein said electrically conductive support member isinsulated from ground.
 6. The invention according to claim 1, wherein aswitch is electrically connected to the electrically conductive supportmember, the switch including at least three positions which comprise: a.means to bias the support member to a first pole of the DC source, b.means to bias the support member to a second pole of the DC source, andc. an open position wherein the support member is insulated from ground.7. A process for the preparation of a web for deposition of a coatingthereon, the web including a non-uniform distribution of electric chargethat is to be substantially removed from the web prior to coating, theprocess including the steps of: a. providing an electrical source of DCpotential, the source having first and second electrodes with the DCpotential applied between the electrodes; b. exposing one side of theweb to the first electrode so as to generally uniformly electricallycharge the said one side; and c. exposing the said one side to thesecond electrode so as to remove the charge imparted to the said oneside in step (b), and the improvement which comprises: d. supporting theweb so that the opposite side thereof is in contact with an electricallyconductive support member that is isolated from ground, said supportmember being charged by said source to a potential between those appliedto said first and second electrodes, whereby said support membercomprises a common counter electrode for said first and secondelectrodes.
 8. The invention according to claim 7 wherein a side of saidweb is a dielectric.
 9. The invention according to claim 8 wherein thenon-uniform distribution of electric charge was placed on the web duringa corona-treatment process.